Composite woods products may be described as falling into five classes, each characterized by the treatment of the wood within the product: plywood, oriented strandboard, particleboard, hardboard, and fiberboard. For each class of composite wood, the configuration of the wood contributes to that product's physical properties and typical application. In addition, the adhesive, the density of the wood, and additives such as resin or fire retardants, may change the characteristics of the product.
Most composite woods are made using a thermosetting or heat curing adhesive to bind or hold the cellulose wood fibers together during the process of manufacturing. Commonly used composite wood adhesives or resin binders are formaldehyde, urea-formaldehyde, melamine-formaldehyde, and isocyanate. Phenol-formaldehyde (PF) resins are typically used for manufacturing products that require some degree of exterior exposure durability. These PF resins are manufactured to be used in high pH composite wood products paste formulations, such as at or above a pH of 10. Urea-formaldehyde resins are typically used in manufacturing products where dimensional uniformity and surface smoothness are of more concern than exterior durability (e.g., particleboard). These resins are manufactured to be used in relatively low pH acidic composite wood products paste formulas. Melamine-formaldehyde is an expensive resin used in decorative laminates or paper treatment. Isocyanates, such as di-phenylmethane di-isocyanate (or MDI) are also used in the manufacture of composite wood products. Natural adhesives, such as tannins, can be modified and reacted with formaldehyde to produce resins, as well. The curing characteristics of the resin, such as the cure temperature and time, are also additional factors in the choice of resin. Urea- and phenol formaldehyde resins are most widely used in manufacturing composite wood products.
These resins are expensive and may cause environmental and health problems. Oil shortages contribute to price increases. Formaldehyde is an irritant, a suspected carcinogen, and the resin base may release volatile organic compounds. (Toxicological Profile, ATSDR, DHHS-PHS 1999). In the composite wood industry, the resin is often extended with other materials that add less expensive solids to the paste formulas. These other solids are of two distinctly different types or classes. For clarity, there are extenders that extend a resin and may add tack or active adhesiveness; this contrasts with fillers, which uniformly lack tack or adhesiveness. Extenders may include proteinaceous and amylaceous materials, which provide the active adhesiveness. See Sellers, T., Jr., PLYWOOD AND ADHESIVE TECHNOLOGIES 451 (1985). Fillers may include lignocellulosic materials, such as corncob or alder bark, which do not provide adhesiveness and are recognized as useful as fillers and not extenders without further treatment. Id.; see, also, U.S. Pat. No. 4,942,191 (Col. 12 list of materials identifying corncob and alder bark as non-adhesive “fillers.”) An active adhesive extender in this context means a substance capable of extending resin and contributes to holding materials together by adhesion. Sellers, T., Jr., PLYWOOD AND ADHESIVE TECHNOLOGIES 610 (1985).
Secondary extenders are generally less expensive materials that also reduce the cost of the resin, reduce the quantity of primary extenders, and the concentration of any deleterious components. Secondary extenders contribute to prepress tack and have some adhesive action, but the resin solids are the ultimate adhesive and moisture resistant binder. Any extender should have the correct viscosity, viscosity stability (long pot life), the ability to develop tack, and a certain stickiness. The endosperm from cereal grain, here referred to as cereal grain flour, or legume flour is often used as a secondary extender. It is the endosperm starch that makes a successful secondary extender. The starch gelatinizes at high pH in a predictable and stable manor, thus supplying the needed paste viscosity properties.
Filler is typically an inert organic fibrous material that does not contribute viscosity, or any binding, or adhesive capability. With regard to high pH PF paste formulations, there are only a select few high fibrous materials that meet these criteria. Examples would include walnut shell, pecan shell, and alder, other tree barks, ground corn cob residue. Most other high fibrous material, include cereal grain milling by-products such as corn bran, sorghum bran, oat hulls, barley hulls, rice hulls, and oil seed and legume processing by-products such as soybean hulls, and cottonseed hulls, and pea hulls, cereal grain by-products such as spent brewers grains expelled or extracted corn germ and malt husks, will swell in the presence of caustic in a high pH phenol formaldehyde formula. The swelling of such materials make them unfit for use as a filler in higher pH adhesive mixtures. However, such materials may work quite well in low pH or acidic paste formulas. It is primarily the fiber in these high fiber materials that does the swelling making them unfit. Attached proteins in some high fibrous materials, especially attached legume protein, also swells in the presence of caustic, further making these high fiber materials unfit to use as a filler in a high pH paste formula.
Some flour extenders known in the field include casein, starch, oil cakes, corn flour, corn gluten protein, seed gums, and primarily wheat flour. Legume flours such as pea, check pea, lentil, broad bean, peanut and soybean might also be considered. These cereal grain and legume flours are the main finished products from the dry milling, wet milling, or bean crushing processes. Secondary extenders made from ground whole cereal grain or legume may contain small amounts of hull and germ which contain most of the fibrous material. Whole grain corn flour may contain approximately 12.4% fiber. Whole pea flour may contain 5.9% fiber (Rodgers table 1). So fiber material in whole grain or legume flour that incorporate the bran and germ world typically be below 15% of total dry weigh composition. Secondary extenders made from whole cereal grain or whole legume, if used as a 100% of the secondary extender, would bring no more than 15% high fiber material to the total weigh of the secondary extender. In some cases, wheat and other flours or materials may be mixed. The combination of gluten or protein and starch in wheat flour appears to aid in the adhesiveness or tack of the final mixture, making it one of the most widely used extenders. Yet wheat flour, although cheaper than resin, is still relatively expensive compared to many other natural alternatives. Wheat flour in the high pH caustic PF resin formula is also somewhat unpredictable or variable. The proteins in wheat flour at high pH can cause stringing and lumping of the adhesive mixture, and accumulation or build-up of glue on spray tips or other equipment, necessitating frequent cleaning. Unmodified native proteins in general can also cause problems with viscosity stability. Unmodified soy protein is particularly unstable at high pH, causing short pot live. Thus, some have sought natural alternatives, such as fiber byproducts mixed with starch, but with limited commercial acceptance.
The term “fiber” is here defined as a polysaccharide that cannot be digested by alpha or beta amylase enzymes. Most commonly in the aforesaid high fiber materials are cellulose, hemicelluloses, and lignin along with certain organic acid. Typical tests to measure the amounts of these material present in high fiber, or fibrous material are crude fiber, acid detergent fiber (ADF), neutral detergent fiber (NDF), and total dietary fiber (TDF). For the purpose herein, total dietary fiber (TDF) is used to define the term fiber. The TDF in high fibrous materials or components can range from 50% to 95%, with 65% to 85% most typical.
The same shortcomings that make the use of most high fiber by-products from cereal grain, oil seed, or other legume processing unacceptable for use as an extender or filler in high pH paste formulas, also make high fibrous materials unacceptable for use as a secondary extender in amount more than about 10% by d.b. weight of the secondary extender. In this context, as noted above, 50% or more TDF may be considered to be a high fiber material or component.
It would be desirable to provide an adhesive, secondary adhesive extenders and mixture thereof for use in high pH PF resin composite wood products satisfactory to the performance and manufacturing needs, as well as a method for the production and use of such an adhesive or adhesive extender in the manufacture of composite wood products. High fiber materials, however, may be inexpensive—which would render their use otherwise attractive, should their drawbacks be overcome. Accordingly, a cost effect and simple approach to enable the use of such materials as extenders is desirable.